Lithium secondary batteries have been widely used as power sources for various types of portable electrical and electronic equipment, as batteries for electric automobiles, and the like.
A lithium secondary battery comprises a positive electrode, a negative electrode, a nonaqueous electrolyte, and normally a separator as well. Developmental improvements for each component are under active investigation.
Normally, to fabricate the positive electrode, for example, a slurry of the positive electrode mixture is prepared by dispersing a positive electrode active material together with a binder, and if necessary a conductive material, in an organic solvent, followed by applying the dispersion to a positive electrode current collector, removing the solvent by drying, and then rolling.
Conventionally, polyvinylidene fluoride (PVdF) has been widely used as a binder for the positive electrode in lithium secondary batteries. Patent Document 1, for example, discloses a technique for fabricating an electrode sheet in which a positive electrode mixture is prepared by mixing a lithium-containing oxide such as LiCoO2 as the positive electrode active material and graphite as the conductive material with PVdF, the resulting mixture is dispersed in N-methylpyrrolidone to make a slurry, and the slurry is applied to an aluminum foil positive electrode current collector; a negative electrode mixture is prepared by mixing a carbonaceous material as the negative electrode active material with PVdF, the resulting mixture is dispersed in N-methylpyrrolidone to make a slurry, and the slurry is applied to a copper foil negative electrode current collector; and after each of the obtained collectors has been dried, the collector is subjected to compression molding with a roller press. Recently, in response to the need for higher capacity and voltage there has been a change of the positive electrode active material from LiCoO2 to ones containing nickel and manganese. However, a positive electrode active material that contains nickel or manganese is more basic than LiCoO2, which disadvantageously makes gelation of PVdF more likely to occur.
In addition, PVdF is more likely to swell in an organic solvent for the nonaqueous electrolyte such as propylene carbonate, ethylene carbonate, diethyl carbonate or a mixture thereof used in lithium ion secondary batteries. This is a problem because the adhesion of the PVdF to the metal foil current collector deteriorates with repeated charge-discharge cycles, resulting in an increase in internal resistance and therefore a decrease in battery performance. In addition, an electrode sheet using PVdF as a binder has poor flexibility, and in the step of folding the electrode sheet 180° to fabricate a prismatic cell or the step of tightly rolling up the electrode sheet to fabricate a cylindrical cell, a problem is likely to occur because the electrode mixture peels off the electrode sheet, resulting in a poorer production yield.
In addition, Patent Document 2 discloses the use of a rubbery elastic material having as the main component thereof a fluorinated binary copolymer such as a vinylidene fluoride (VdF)/hexafluoropropylene (HFP) copolymer or VdF/chlorotrifluoroethylene (CTFE) copolymer as a binder to impart adhesiveness to the positive electrode active material in order to counter the expansion and contraction thereof during charging and discharging in a secondary battery with a nonaqueous electrolyte. However, such a copolymer has poorer crystallinity than PVdF and swells as much or more than PVdF in an organic solvent for the nonaqueous electrolyte. Hence, depending on the type of electrolyte, the polymer dissolves away and cannot fulfill its role as a binder. In addition, the polymer is also prone to gelation with a positive electrode active material containing nickel or manganese.
As a similar binder Patent Document 3 discloses the use of a fluorinated copolymer mainly comprising VdF, tetrafluoroethylene (TFE) and HFP in place of PVdF as a binder. The compositional ranges of the copolymer components disclosed in the claims of Patent Document 3 are 0.3-0.9 VdF, 0.03-0.5 HFP, and 0-0.5 TFE in molar ratio, and the total molar ratio of these 3 monomers ranges from 0.80 to 1. Patent Document 3 also points out that PVdF causes problems in the production process because it will only dissolve in specific solvents that are extremely polar, have a high boiling point, and may also be toxic such as the aforementioned N-methylpyrrolidone, dimethylacetamide, dimethylformamide, and methyl sulfoxide. Therefore, for example, when applying an active material using said solvent followed by molding in the process of manufacturing an electrode, the high-boiling point solvent takes too long for drying, and sealed equipment, ventilation systems, and the like must be installed because of the toxicity thereof. Therefore, in Patent Document 3 the aforementioned copolymer that dissolves in a low-cost, low-boiling point, common organic solvent, e.g., a ketone such as methyl ethyl ketone or methyl isobutyl ketone; an ester such as ethyl acetate or butyl acetate; an ether such as a dioxane or tetrahydrofuran; or a mixture thereof is used to solve the above problems. Just as in the case of the aforementioned VdF/HFP binary copolymer and VdF/CTFE binary copolymer, however, basically this VdF/TFE copolymer also swells considerably in an organic solvent for the nonaqueous electrolyte, and therefore problems such as a decline in battery characteristics can occur because during the long-term use of the battery the electrode mixture peels off the current collector and the active material is lost therefrom.
To overcome these problems Patent Document 4 discloses a binder that is soluble in particularly common solvents and does not easily swell in an organic solvent for the electrolyte. The binders disclosed in Patent Document 4 are a fluorine-containing binary copolymer consisting of 50 to 80 mol % of VdF and 20 to 50 mol % of TFE, and a fluorine-containing ternary copolymer consisting of 50 to 80 mol % of VdF, 17 to 50 mol % of TFE, and less than 3 mol % of a different comonomer. Patent Document 4 discloses a VdF/TFE copolymer and a VdF/TFE/HFP copolymer as the VdF/TFE copolymers used in the examples.
At the same time, various developments in the positive electrode active material have proceeded from the standpoint of battery characteristics and safety, the depletion of resources (rare metals), and the like, and most recently a positive electrode active material has appeared that contains nickel or manganese and reduces the content of the rare metal cobalt. However, because such a positive electrode material containing nickel or manganese is more basic, the slurry is prone to gelation.
Furthermore, when a positive electrode mixture slurry is prepared using a positive electrode material that contains nickel or manganese, and a PVdF binder, and then the slurry is applied and dried, the binder and positive electrode material will react as they are heated, even at temperatures of about 100° C. This is considered to be a cause of a loss in positive electrode capacity, hardening of the electrode with resulting cracking, etc.
Patent Document 1: JP H4-249859 A
Patent Document 2: JP H4-95363 A
Patent Document 3: JP H8-4007 B
Patent Document 4: JP H10-233217 A